1. Field of the invention
This invention relates generally to an automatic circuit breaker for a small-sized motor, and more particularly to an automatic circuit breaker for a small-sized motor which is designed to open the motor power circuit to prevent reverse battery charging caused by unequal discharging of two or more batteries used in series connection as a power source for the motor. This invention can be applied to model planes, model ships and model automobiles to prevent reverse charging which tends to occur especially when rapidly rechargable Ni-Cd batteries are used as a power source.
2. Description of the prior art
A rapidly rechargeable Ni-Cd battery, whose discharge current is extremely high because of its low internal resistance, has recently been developed and is now widely being used in various battery-operated electric appliances.
When more than two batteries of this type, though not limited to Ni-Cd batteries, are used in series connection, the service life of the batteries tends to be substantially reduced by the so-called reverse charging in which one of the batteries is completely discharged earlier than the other batteries, thereby imposing a load on the other batteries. Thus, it is desired to open the power circuit, when the remaining charge in the batteries drops to a certain level to prevent complete discharge of the batteries.
Such consideration is especially important in a model plane or model ship powered by batteries since such a model often travels out of the reach of the operator.
Conventional automatic circuit breakers for this purpose employ, for instance, a method to open the power circuit by actuating a relay when the voltage drops, a method using a semiconductor and a relay, or a method to break the power circuit, in the case of a model plane, by actuating a switch by means of part of the propeller shaft which is rotated by wind force as the motor speed drops due to reduced battery voltage. All of these methods have achieved satisfactory results in specific applications. However, this invention is intended to achieve these results by providing a novel automatic circuit breaker, in place of the aforementioned conventional methods, to prevent reverse charging by sensing the reaction torque at a bearing of a motor powered by batteries.